Membrane for capacitive vacuum measuring cell

ABSTRACT

A ceramic membrane for a capacitive vacuum measuring cell includes a thin ceramic membrane with a thickness of &lt;250 μm, in particular less than 120 μm. The membrane is produced from a ribbon-shaped green body of Al 2 O 3 , and is given high planarity by smoothing the membrane after sintering. The green body is sintered at a sintering temperature that is higher than the smoothing temperature applied following sintering.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application is a divisional application based uponco-pending U.S. patent application Ser. No. 09/218,934, filed Dec. 22,1998, soon to be issued as U.S. Pat. No. ______, and the benefit of suchearlier filing date is hereby claimed by Applicant under 35 U.S.C. §120.In turn, parent application Ser. No. 09/218,934 claims foreign priorityrights, under 35 U.S.C. §119, on the basis of prior-filed Swiss patentapplication Ser. No. 1997 2954/97, filed Dec. 23, 1997, and the benefitof such foreign priority date is also claimed by Applicant herein.

BACKGROUND OF THE INVENTION

[0002] The present invention generally relates to a membrane for acapacitive vacuum measuring cell.

[0003] It is known that pressures or pressure differences can bemeasured by applying pressure to a thin membrane and measuring itsdeflection. A known and suitable method for measuring the deflection isto design the membrane arrangement as a variable electrical capacitance,where the capacitance change which correlates with the pressure changeis evaluated by measurement electronics in the known manner. Thecapacitance is created by arranging a thin, flexible membrane very closeto another surface and by depositing an electrically conductive film onboth mutually opposed surfaces or by fabricating them from electricallyconductive material. When pressure is applied to the membrane, thedeflection changes the distance between the two electrodes which leadsto an analyzable capacitance change of the arrangement. Sensors of thistype are mass-produced from silicon. The flat basic body, as well as themembrane, often consist entirely of silicon. There are also versionsthat are made of composite materials such as silicon with a glasssubstrate. Such sensors can be produced very economically. However, invacuum applications, pressure sensors of this type are normally usableonly for higher pressures in the range of approx. 10⁻¹ mbar to severalbar. High resolution at pressures below 10⁻¹ mbar is no longerachievable with silicon. One of the reasons for this is that the siliconsurface reacts with the environment, which impairs the sensitive sensorcharacteristic. Already water vapor that forms part of normalatmospheric air leads to a corresponding reaction on the surfaces. Theproblem becomes even more serious when the sensor is used in chemicallyaggressive atmospheres. For this reason, attempts were made to protectsuch silicon sensors against external influences by passivating thesurfaces. Attempts were also made to deposit protective coatings on thesurfaces in order to improve the durability and the resistance againstchemically aggressive environments as described in DE 41 36 987. Suchmeasures are costly and, in the case of mechanically deformable partssuch as membranes, have only limited success, in particular in highlyaggressive media such as fluorine, bromic acid and their compounds whichare typically used in vacuum etching processes.

[0004] For this reason, attempts were made to build vacuum measuringcells entirely from corrosion resistant materials such as Al₂O₃. A knownarrangement of this type is shown in FIG. 1. The vacuum measuring cellconsists of a ceramic plate (20) above which a membrane (22) is arrangedwith a small gap between the two of them and a fusible seal (21) betweenthe ceramic plate (20) and the edge of the membrane. The ceramic plate(20) together with the membrane (22) forms a reference vacuum chamber(25) that is evacuated down during the manufacturing process through apumping port and which is sealed with a seal (28). The mutually opposedsurfaces of the ceramic plate (20) and the membrane (22) inside thereference vacuum chamber (25) are coated with electrically conductivematerial and connected to insulated external terminals on which thecapacitance signal can be evaluated by means of an electronic device(not shown in the illustration). To achieve corrosion resistance, plate(20) and membrane (22) are both made of ceramic material such as Al₂O₃.This vacuum measuring cell in turn is arranged in a vacuum-tight housing(23) that features a port (24) which is connected to the media to bemeasured. Via port (24) of the vacuum measuring cell, the resultingmeasurement vacuum chamber (26) is sealed off against the membrane (22)by means of an elastomer seal (27) so that the pressures to be measuredreach only the surface of the membrane (22). For the purpose of sealing,the entire cell is pressed via the ceramic plate (20) and membrane (22)against the elastomer seal (27). Up to now, vacuum measuring cells ofthis type have been usable only for higher pressures in the range of 0.1mbar to 100 bar. In addition, this design leads to stress in thematerials which, at lower pressures, for example <1 mbar, significantlyimpairs the reproducibility of measurement results and the resolution.The ceramic membranes (22) used so far have a thickness ranging from 279μm to 2540 μm. Such designs are not suitable for achieving widemeasurement ranges, in particular low pressures of 0.1 mbar to 10⁻⁶mbar. In addition, designs of this type, as disclosed also in U.S. Pat.No. 5,553,502, are costly.

[0005] The objective of the present invention is to eliminate thedisadvantage of the current state of the art. In particular, theobjective of the present invention is to implement an easy-to-produce,economical membrane consisting of Al₂O₃ for a vacuum measuring cell thatis suitable for measuring pressures from 10⁻⁶ mbar to 1000 mbar, inparticular from 10⁻⁶ mbar to 1 mbar, with an accuracy of better than 1%,preferably better than 0.3% of the measured value. The measurement rangecan be covered or subdivided into several vacuum measuring cells ormembrane versions according to the invention. In addition, this vacuummeasuring cell shall be corrosion resistant to aggressive media, have acompact design, and be economical to manufacture.

SUMMARY OF THE INVENTION

[0006] With respect to the generic membrane, the problem is solvedthrough the characteristic features of patent claim 1. The dependentpatent claims relate to preferable other versions of the invention.

[0007] The capacitive membrane according to the invention is madeentirely out of ceramic, in particular Al₂O₃. In consequence, it is nowpossible to build a vacuum measuring cell entirely out of corrosionresistant material, preferably out of Al₂O₃. This results in highcorrosion resistance and long term reproducibility. Only in the areaswhere sealing is required, or where feedthroughs are provided, are smallamounts of materials other than Al₂O₃ used, if the Al₂O₃ is not fusedwithout addition of the foreign material. A vacuum measuring cellconsists of a first plate-shaped housing body above which a membrane,sealed along its edges, is arranged so that it encloses a referencevacuum chamber. On the side pointing away from the reference vacuumchamber, there is a second housing body, also sealed along its edges, sothat a measurement vacuum chamber is formed there. This measurementvacuum chamber features a port for connecting the medium to be measured.The surface of the first housing body and the membrane that form thereference vacuum chamber are coated with an electrically conductivefilm, for example, gold, and constitute the electrodes of thecapacitance measuring cell. The electrodes are led out, for example,through the first housing body or through the sealing area in the edgezones. The essentially parallel electrode surfaces are spaced apart from2 μm to 50 μm. Sealing of the membrane in the edge zone against the twohousings is preferably achieved through welding, for example, laserwelding. Highly suitable, and simple to use, is also a glass brazingmaterial that is corrosion resistant. Another possibility of achieving asealing bond is to connect the housing parts diffusively, for example,in the green body state in which the objective is to completely avoidmaterial other than Al₂O₃.

[0008] The membrane according to the invention in the measuring cellmentioned above essentially allows a symmetric design that avoids allstress in the housing. This is particularly important in order toachieve high measurement sensitivity combined with high measurementaccuracy and reproducibility at low pressures. It also allows theutilization of a very thin ceramic membrane according to the invention,which is essential for reliably measuring vacuum pressures lower than100 mbar, and in particular lower than 10 mbar, by means of capacitive,all-ceramic measuring cells. For this purpose, membrane thicknesses of10 μm to 250 μm are needed, where membrane thicknesses of 10 μm to 120μm are preferred in order to achieve a very good resolution. Typicalmembrane thicknesses are, for example:

[0009] at 1000 Torr: membrane thickness 760 μm±10 μm

[0010] at 100 Torr: membrane thickness 345 μm±10 μm

[0011] at 10 Torr: membrane thickness 150 μm±10 μm

[0012] at 1 Torr: membrane thickness 100 μm±10 μm

[0013] at 0.1 Torr: membrane thickness 60 μm±10 μm

[0014] at 0.01 Torr: membrane thickness 40 μm±10 μm

[0015] Such thin membranes are very difficult to manufacture and, afterthe sinter step, require at least one additional smoothing step. It isalso very important for the membrane to be sufficiently helium tightwhich can be achieved only if the grain size of the membrane material isnot too large and remains within the range of <20 μm. Smaller grainsizes of <10 μm are preferred, in particular, those <5 μm. In all cases,the cross-section of the membrane viewed across the thickness shouldcontain at least two grains; the membranes are particularly tight ifmore than five grains are on top of each other.

[0016] Another important criterion for achieving an accurate measuringcell is the planarity of the membrane surface. The unevenness across theentire surface should in all cases not exceed 30% of the electrode gap,preferably no more than 15%. This means that the unevenness across theentire surface should not exceed 10 μm, preferably not more than 5 μm.The unevenness is defined here as the difference between the lowest andthe highest point. To achieve the desired long-time stability, thepurity of the aluminum oxide used for the membrane should be at least94%, with preferred values being above 99%.

[0017] To ensure that the quality of the membrane seal in the edge zoneis not impaired, it is advantageous to lead out the electricallyconductive layers via feedthroughs that are arranged on the firsthousing body, rather than directly through the membrane seal or fusedjoint.

[0018] To ensure accurate functioning of the measuring cell over a longperiod of time, the reference vacuum chamber must have a high-qualityvacuum with long-time stability. After evacuation, a getter should beprovided that is preferably arranged in a small volume on the. firsthousing and communicates with the reference vacuum chamber. This getterensures that the reference vacuum pressure is lower, preferably by atleast one decade, than the pressure to be measured. To preventcontaminations of the internal measuring cell space, a getter type thatis not evaporating should be chosen.

[0019] Measuring cells designed according to the invention can be verycompact and economical to produce. The diameter of such a cell can bebetween 5 and 80 mm where the measuring cell preferably has a diameterof 5 to 40 mm. The thickness of such a cell is preferably in the rangeof 2 mm to 25 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a prior art drawing of a known vacuum measuring cellbuilt mainly from corrosion resistant materials such as Al₂O₃.

[0021]FIG. 2 shows a schematic cross-section of a capacitive vacuummeasuring cell according to the invention.

[0022]FIG. 3 shows an enlarged cross-sectional detail according to FIG.2.

[0023]FIG. 4 shows a temperature/time diagram for the sintering step ofthe membrane.

[0024]FIG. 5 shows a temperature/time diagram for a smoothing step ofthe membrane.

[0025]FIG. 6 shows a membrane resting between two flat plates, andcompressed therebetween.

[0026]FIG. 7 shows a membrane cast from a thin strip of Al₂O₃ green bodysupported on a carrier foil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] For manufacturing a functional measuring cell that possesses theaforementioned characteristics, the specifications of the correspondingmanufacturing process must be closely followed. In particular, themanufacture of thin ceramic membranes requires special care. Themembrane as well as the complete unit should be entirely free ofinternal stress.

[0028] Suitable Al₂O₃ membranes according to the invention aremanufactured by first mixing a slurry according to a specific recipe,and by thinly and evenly spreading the doughy mass on a strip shapedcarrier material, for example, a plastic foil. After drying, theselayers are inspected for irregularities such as bubbles or pits. Thismass, which is not sintered yet, is referred to as the green body. Thedesired membrane shape is cut out of the strip shaped green body 106(see FIG. 7) material, after which the material is still sticking to theplastic foil 108 (see FIG. 7). For cutting, tools such as knives orpunching tools are used, or a laser. Cutting or scoring of the greenbody requires particular care that no dislocations or warping againstthe surfaces of the future ceramic membrane occur, as this alsoinfluences the degree of surface unevenness. If a cutting knife is used,a pressing wheel can be applied on the membrane side which preventsundue warping of the green body. Subsequently the preferably circularcut membranes are separated from the foil strip by drawing off thelatter, for example, across an edge. The membranes are subsequentlysintered in a furnace. For sintering, the membranes are preferablyplaced on hard-sintered, flat Al₂O₃ plates that can be on top of eachother, and sintered typically at 1630° C. The temperature is graduallyraised to 1630° C. over a period of approx. 400 minutes, whichcorresponds to a temperature rise of about 4° C. per minute. Thetemperature is then held for a few minutes at this level, for example, 6minutes, and then slowly decreased at the rate of about 3° C. per minuteover 210 minutes to 1000° C., and in a second step, with a temperaturereduction of about 6° C. per minute over about 170 minutes, to roomtemperature. The result is a ceramic membrane which, in contrast to thegreen body, has a hard pure ceramic structure, and the additives of thegreen body material have evaporated. After this sintering step, themembrane is very uneven and, at a diameter of 40 mm, has a warpage ofseveral millimeters.

[0029] In this condition, the membrane cannot be used due to the strongwarpage and internal stress in the material. The membrane must besmoothed in at least one additional step. For this purpose, the membraneis again heated in the furnace. The membrane is carefully sandwichedbetween massive and highly planar, hard-sintered Al₂O₃ plates (also“dead”, that is, large-grained Al₂O₃) which, for a membrane diameter of40 mm, have a weight of several 10 to several 100 grams, or in theexample about 60 grams, or are correspondingly weighted down. Thetemperature is slowly increased at 4° C. per minute over 390 minutes toapproximately 1570° C. After a short dwell time of several minutes,approximately 25 minutes at this temperature, the temperature is loweredslowly at approx. 5° C. per minute over approx. 115 minutes until 1000°C. are reached. Subsequently, the temperature is lowered at approx. 6°C. per minute over about 166 minutes until ambient temperature isattained. After this smoothing step, the membrane has only a very smallamount of residual warpage of a few tenths of a millimeter. Important inthis smoothing step is that the temperature is not raised as high as inthe first sintering process, preferably up to a temperature which is atmost 100° C. lower than the sintering temperature. To achieve excellentresults required for the measuring cell to be built, this smoothingheating step must be performed at least twice. For reasons of economy,these smoothing steps should be performed in such a way that no morethan two such steps are needed. Particularly good results are achievedwhen the membrane is carefully separated from the plate between heatingsteps and redeposited in a slightly offset position. Preferably, themembrane is even placed upside down. The utilization of a stack ofseveral flat plates with membranes sandwiched in between is particularlyeconomical.

[0030] The functionality of measuring cells designed as described aboveis decisively influenced by the membrane. Said manufacturing processallows the production of thin membranes with high density and goodplanarity. Strict adherence to the corresponding parameters during thesintering and subsequent smoothing steps is essential. During sintering,maximum temperatures from 1300 to 1800° C., preferably 1400 to 1700° C.,must be attained. This maximum temperature should be achievable at leastbriefly, but maintained in this range no longer than 180 minutes. Theheat-up rate should not exceed 25° C. per minute. Preferably, theheating phase is subdivided into two steps: After a temperature of 1000to 1300° C. has been attained, further heating to the final temperatureshould take place at a maximum of 15° C. per minute. When said maximumtemperature has been attained and the dwell time maintained, themembrane is cooled off again at a rate of no more than 25° C. perminute. If the heating and/or cooling is performed too quickly, themembranes become wavy and porous. Longer times are not harmful, but areuneconomical.

[0031] During the smoothing step, the same conditions have to bemaintained as in the sintering step, and the smoothing temperature isnot to exceed the sintering temperature at any time. Preferentially, themaximum smoothing temperature should stay at most 100° C. below themaximum sintering temperature.

[0032] Membranes are now available that have selectable thicknesses inthe range of 10 μm; to 250 μm, preferably <120 μm. With the processdescribed above, membrane planarities can be achieved that are betterthan 10 μm across the entire surface, preferably even better than 5 μm.The mean grain size in the membrane material is less than 20 μm,preferably less than 10 μm, and even less than 5 μm is achievable. Thismeans that the requirement that at least 2 grains, preferably at leastfive grains, exist across the thickness, can easily be achieved. In thisway, helium tight membranes, as required for vacuum measuring cellapplications, can be produced. The membrane is now ready to be used forbuilding the measuring cell.

[0033] The membrane, as well as a flat surface of the first housing bodymade of Al₂O₃, are now coated with an electrically conductive film forcreating the electrodes. For example, a metallic paint, for example apaint containing gold, can be used which, for example, is brushed orsprayed, preferably printed on. Another method is to create theelectrically conductive layer by means of evaporation coating,preferably by sputter coating. To allow the deposition of an accurateand defined film, it is advantageous if, for example, a gold layer thatinitially is deposited with a relatively large thickness of about 1 μm,is subsequently thinned down in the inner area to approx. 5 nm by meansof an etching process, preferably an ion etching or sputter etchingprocess. In this way, a thicker edge area is created which cancompensate diffusion losses if, for example, a brazing step isperformed. A preferred process that is simple to handle in practice isto first deposit a thin layer of several nm across the entire surfaceand subsequently a thicker layer of gold at the edge by means of screenprinting (that is, a combination process and different filmthicknesses). Membranes or housings processed in such a way aresubsequently tempered at temperatures of several 100° C., preferably inthe range of 650° C.

[0034] The second ceramic housing which is arranged on the measurementside consists of a flat ceramic plate which, on the membrane side, canhave a flat recess in order to form a sufficiently large vacuum chamber.The connection port is connected to this ceramic housing by means ofbrazing, bonding or gluing, preferably by means of glass brazing, insuch a way that the connection opening can communicate with the futuremeasurement vacuum chamber.

[0035] In the peripheral area where the seal is created, the membrane iscoated on both sides with a glass paste, preferably by means of screenprinting. After drying, the membrane with the glass paste is baked in anoven at several 100° C., preferably at about 670° C. Subsequently, theglass surface is polished on both sides and, thereby preferably also,the future electrode spacing is defined.

[0036] With the aid of said coating process, the upper ceramic housingon the electrode side can, on the external surface, additionally becoated with an electrically conductive film in order to achieveshielding. Also here, the connection points are formed on the housing.In an additional step, the drill holes for the electrical feedthrough ofthe electrode connections are metallized, preferably with silver.

[0037] In a test phase, the first housing with the electrode and thefeedthroughs, together with the deposited membrane, is checked fortightness and for correct electrode distance. Subsequently, the lowerhousing part is mounted, and the entire assembly is loaded with weightin order to test the function and distances. In a mounting frame, thegetter connection may additionally be mounted, and under a load weightof about 200 grams, the glass seals are baked at several 100° C.,preferably about 630° C. Subsequently, a test is performed to check thatthe required distances are maintained. If necessary, the membranespacing can be corrected through additional weight loading or relieving,and an additional firing process. The sealing process must be executedvery carefully and, as mentioned, no stress should occur in themeasuring cell arrangement. Alternatively also, direct bonding can beused in place of glass or other sealants, preferably laser bonding.

[0038] The various features of novelty which characterize the inventionare pointed out with particularity in the claims annexed to and forminga part of this disclosure, and are entirely based on the Swiss priorityapplication no. 1997 2954/97 filed Dec. 23, 1997.

[0039] The invention is described schematically based on the followingillustrations which serve as examples:

[0040]FIG. 2 shows a schematic cross-section of a capacitive vacuummeasuring cell according to the invention.

[0041]FIG. 3 shows an enlarged cross-sectional detail according to FIG.2.

[0042]FIG. 4 shows a temperature/time diagram for the sintering step ofthe membrane.

[0043]FIG. 5 shows a temperature/time diagram for a smoothing step ofthe membrane.

[0044] A capacitive measuring cell made of a membrane of Al₂O₃ accordingto the invention with a structure essentially symmetrical about themembrane is illustrated by the cross-section in FIG. 2. The firsthousing (1) consists of a ceramic plate made of Al₂O₃ which along itsedges is tightly bonded at a distance of 2 μm to 50 μm relative to theceramic membrane (2) and which encloses a reference vacuum (25). Thedistance between the two surfaces is usually established directly duringthe assembly by means of the sealing material (3) located between themembrane edge and the housing. In this way, a completely plane housingplate (1) can be used. In the same way, a measurement vacuum chamber(26) is formed in a second housing (4) on the opposite membrane side;this vacuum chamber is accessible for the media to be measured via aconnecting port (5) through an opening in the housing (4).

[0045]FIG. 3 shows an enlarged cross-sectional detail of the edge zoneof a measuring cell. The seal (3) on both sides of the membrane (2)defines, as mentioned above, the distance of the two housings (1 and 4).This seal consists, for example and preferably, of glass paste that iseasy to handle and can, for example, be applied by means of screenprinting. In a typical measuring cell with an external diameter of 38 mmand a free internal membrane diameter of 30 mm, the distance (3) isapprox. 2 to 50 μm, preferably 12 to 35 μm. In this example, the firsthousing (1) has a thickness of 5 mm, and the second housing (4) athickness of 3 mm. The inner area of the second housing (4) ispreferably designed with an approx. 0.5 mm deep recess, as shown in FIG.2, in order to enlarge the measurement vacuum chamber (26). On thereference vacuum side, the membrane (2) and the housing (1) are eachcoated with an electrically conductive film (7). These two films are notelectrically interconnected. Films (7) can, for example, be painted on,printed on, sprayed on, or be deposited by means of a vacuum process.Preferably, they are deposited by a vacuum process such as byevaporation coating or sputtering. Particularly suited as a filmmaterial is gold, which is deposited, for example, with a film thicknessof 1 μm and is subsequently thinned down to a few nanometers, preferablyto 5 nm, by means of sputter etching. In this way, the film thicknesscan be defined so that it is thin enough and is free of stress. Theelectrical connections of the membranes (7) are preferably establishedwith vacuum-tight, electrically conducting feedthroughs (6), preferablythrough the housing (1) where they can subsequently be connected to theevaluation electronics. The evacuation line which leads through thefirst housing plate (1) and the getter arrangement are not shown in FIG.3.

[0046] An example of an optimized time/temperature profile for themembrane sintering step, beginning with the green body, is shown in FIG.4. Important for the sintering process is that the membrane materialafter sintering has a mean grain size not greater than 20 μm, preferablynot greater than 10 μm, preferably smaller than 5 μm in order to achievehigh gas or helium tightness of the thin membrane. In the cross-section,across the thickness of the membrane, at least 2 grains, preferably 5grains, should be present. After the sintering step, the membrane isbuckled by several mm relative to a plane surface. In this condition,the membrane is not usable and must be smoothed; that is, planaritiesmust be achieved that deviate by no more than 10 μm from the ideal planesurface, preferably less than 5 μm. This is achieved by carefullyheating the membrane which is deposited on a plane surface, in a furnaceup to the softening point. Already due to its own weight, it will adaptitself to the plane base. To achieve better results, this process can berepeated, and the membrane 2 (see FIG. 6) can additionally be smoothedby sandwiching it between flat plates 100 and 102 (see FIG. 6). Aparticularly simple solution is to press the membranes with the weightof the plates themselves, which is in the range of several 10 grams.Between the heating steps performed for the purpose of smoothing, themembranes should be detached from the flat plates and redeposited in adifferent direction or upside down. As a rule, two smoothing steps aresufficient and should be attempted for reasons of economy.

[0047] An example of an optimized time/temperature profile for asmoothing step, starting with a sintered membrane, is shown in FIG. 5.In this step, the membranes reach a maximum temperature of 1570° C. andremain below the sinter temperature of 1630° C. The membrane becomessoft and can adapt itself to the plane base, either through its ownweight or by means of weight loading.

I claim:
 1. A membrane for a capacitive vacuum measuring cell, saidmembrane being made of a membrane material consisting of Al₂O₃ andhaving a thickness within the range of 1 μm to 250 μm.
 2. The membranerecited by claim 1 wherein said membrane has a thickness within therange of 10 μm to 120 μm.
 3. A membrane according to claim 1 wherein themembrane material has a mean grain size of ≦20 μm.
 4. The membranerecited by claim 3 wherein the membrane material has a mean grain sizeof ≦10 μm.
 5. The membrane recited by claim 4 wherein the membranematerial has a mean grain size of ≦5 μm.
 6. A membrane according toclaim 1 wherein the membrane has a thickness cross-section, and whereinat least two grains of membrane material are present in the thicknesscross-section of the membrane.
 7. The membrane recited by claim 6wherein at least five grains of membrane material are present in thethickness cross-section of the membrane.
 8. A membrane according toclaim 1 wherein the membrane has a surface, and wherein any unevennessof the membrane surface across the entire surface thereof is not greaterthan 10 μm
 9. The membrane recited by claim 8 wherein any unevenness ofthe membrane surface across the entire surface thereof is not greaterthan 5 μm.
 10. A membrane according to claim 1 in which the purity ofthe Al₂O₃ from which the membrane is made is at least 94%.
 11. Themembrane recited by claim 10 wherein the purity of the Al₂O₃ from whichthe membrane is made is at least 99%.
 12. A membrane according to claim1 wherein the membrane has a generally-circular shape characterized by adiameter, and wherein the diameter of the membrane is within the rangeof 5 to 80 mm.
 13. The membrane recited by claim 12 wherein the diameterof the membrane is within the range of 5 to 40 mm.
 14. A membraneaccording to claim 1 wherein said membrane has at least one generallyplanar surface, and in which said at least one surface includes anelectrically conductive coating.
 15. A membrane for a capacitive vacuummeasuring cell, said membrane being produced by the process of: a.forming the membrane from an Al₂O₃ slurry; b. heating the membrane afirst time in a furnace at a first temperature to sinter the membrane;c. cooling the membrane following heating step b; d. heating themembrane a second time in a furnace at a second temperature to smooththe membrane, the second temperature being lower than the firsttemperature; and e. cooling the membrane following heating step d.